1. Field of the Invention
This invention relates to an exhaust emission control device for removing particulate from an exhaust stream discharged from an internal combustion engine, and more particularly to a reverse cleaning regeneration type exhaust emission control device capable of continuously conducting regeneration of filters for the removal of particulate and a method of regenerating the same.
2. Description of Related Art
Recently, there becomes a problem that particulate emissions including in an exhaust stream discharged from an internal combustion engine for use in vehicles such as truck, bus and the like or construction machines harmfully exert upon environment and human body.
Therefore, an exhaust emission control device provided with a filter is arranged in an exhaust emission path extending from the internal combustion engine to catch and remove particulate included in the exhaust emission through the filter.
In the exhaust emission control device, it is necessary to take out the caught particulate from the filter and regenerate the filter. As a method of regenerating the filter, there has hitherto been known a method of burning out the particulate by directly feeding heat to the filter. In this method, however, there was caused a problem that the filter was fused by combustion heat of the particulate or broken by heat stress.
As a method of regenerating the filter without burning the particulate, there is known a reverse cleaning regeneration system in which a high-speed gas stream is supplied in a direction opposite to the flowing of the exhaust stream to remove out the particulate from the filter and then the removed particulate is burnt out in another container by using an electric heater.
When using such a reverse cleaning regeneration system, it is not required to directly feed high heat for regeneration to the filter, so that the breakage of the filter due to heat stress is not caused, and ash is not deposited on the filter, and the clogging in the filter is hardly caused.
However, if such a series of actions is carried out in a single filter, a back pressure in the exhaust emission path rises in the course of removing the particulate, which badly affects the operability of such a system.
For this end, there is proposed a method of using a pair of filters and alternately regenerating them (Nippon Hakuyo Kikan Gakkaishi, vol. 27, No. 6, pp446-452).
In the latter method, as shown in FIG. 1, the reverse cleaning regeneration type exhaust emission control device comprises a pair of filters 1 and 4, in which exhaust emission (shown by arrow) passes through the filters 1 and 4 at a steady state. If it is intended to regenerate the filter 1, as shown in FIG. 2, a valve 2 disposed between the filter 1 and a discharge port is closed to prevent the flowing of air for regeneration to the discharge port. Then, as shown in FIG. 3, compression air is jetted through a nozzle 3 in a moment to conduct the removal of the particulate caught by the filter 1 and the regeneration of the filter 1. After the regeneration of the filter 1, as shown in FIG. 4, the valve 2 is again opened to conduct the usual removal of the particulate using the two filters 1 and 4.
During the above regeneration of the filter 1, the exhaust emission always passes through the filter 4, so that the removal of the particulate is continuously conducted.
The regeneration of the filter 4 is carried out in the same manner as in the regeneration of the filer 1 by using a valve 5 and a nozzle 6.
The above exhaust emission control device using the two filters can continuously conduct the reverse cleaning while continuing the removal of the particulate, so that it is possible to apply such a device to trucks running over a long distance and an internal combustion engine continuously operating in an unventilatable place for a long time.
Moreover, the working of the conventional reverse cleaning regeneration type exhaust emission control device was stopped by stopping the flow of the exhaust emission accompanied with the stop of the internal combustion engine. As a result, the burning of the particulate removed out from the filter is frequently stopped on the way, so that when the flowing of new exhaust emission is started at a state of leaving the particulate, there are caused problems that the efficiency of removing the particulate becomes bad and the pressure loss undesirably rises and the like.